(i) Field of the Invention
The present invention relates to a lock-up clutch controller for controlling the lock-up clutch of a fluid coupling of an automotive automatic transmission.
(ii) Description of the Prior Art
It is desirable to engage a lock-up clutch for locking a fluid coupling gradually with a limited slip in coupling directly the input and output shafts of the fluid coupling, otherwise undesirable shock will be produced. In view of such requirement for the performance of the lock-up clutch, the inventors of the present application proposed a direct-coupling clutch control mechanism for an automotive automatic transmission in Japanese Patent Application No. 180870/1983. The direct-coupling clutch control mechanism proposed in this prior application was intended to control a direct-coupling clutch provided in a fluid coupling, which clutch includes a frictional coupling surface connected to the input shaft and a clutch disk connected to the output shaft and adapted to engage when working fluid is supplied (1) from a second fluid line connected to the interior of the fluid coupling at a point other than the gap between the frictional coupling surface and the clutch disk and (2) to a first fluid line connected to the gap and to disengage when the working fluid is supplied from the first fluid line to the second fluid line. The control mechanism was provided with a direct-coupling clutch control valve for either (1). Connecting the second fluid line to a fluid source and connecting the first fluid line to a first draining line in order to make the direct-coupling clutch engage or (2) connecting the first fluid line to the fluid source and connecting the second fluid line to a second draining line in order to make the direct-coupling clutch disengage, according to the running condition of the vehicle. That direct-coupling clutch provided for gradual engagement by limiting the flow rate of the working fluid with a flow rate control mechanism provided in the first draining line. However, since this direct-coupling clutch control mechanism employed orifices for regulating the fluid pressure in disengaging the lock-up clutch, the pressure regulating characteristics were unvariable, and hence the lock-up clutch could not engage smoothly and sometimes shock was produced. When the pressure was regulated to enable smooth engagement of the direct-coupling clutch, the pressure within the fluid coupling dropped and cavitation was likely to occur. Furthermore, when a first spool valve for changing the connection of the clutch disengaging circuit and the clutch engaging circuit to the pressure source or to the drain line and a second spoolvalve for regulating the speed of variation of the fluid pressure difference between the clutch disengaging circuit and the clutch engaging circuit were provided, the first and second spool valves required separate control means, thus complicating the control circuit and procedures.